Electro-mechanical systems have a dynamic behavior that is often dependent on temperature. Also, the dynamic behavior of these systems frequently changes over time. One class of electro-mechanical system known as laser angular rate sensors exhibit system dynamics that are also temperature and age dependent. One component of the laser angular rate sensor known as the path length controller is also affected by these changes.
Laser angular rate sensors, sometimes referred to as laser gyros, are well known in the art. A detailed description may be found in the "Background of the Invention" of U.S. Pat. No. 4,597,667, which is incorporated herein by reference. FIG. 1 shows schematically a laser gyro. Briefly, such sensors include a ring laser gyro 200 supported in a gyro block 5 having a plurality of gas containing tunnels (not shown). At the intersection of the tunnels are mirrors 13, 15, and 18 that define a closed-loop optical path 16 which is traveled by counter-propagating laser beams therein. Practical embodiments of laser angular rate sensors usually include a path length control (PLC) apparatus. The purpose of the path length control apparatus is to maintain a constant path length (usually multiples of the laser wavelength) for the counter-propagating laser beams. Maintaining a constant path length avoids false rotation errors from the laser gyro. The path length control function is usually provided by an arrangement wherein at least one of the mirrors is attached to a piezoelectric transducer which controls translational movement of the mirror. This transducer effects the laser beam path length in response to a drive signal provided through a transducer drive amplifier. In the example shown in FIG. 1 mirrors 13, 15 are path length control mirrors. Mirrors 13, 15 move along paths represented by lines 12, 14 in response to path length control signals.
One technique for maintaining a constant path length is detecting the intensity of one or both of the laser beams and controlling the path length of the ring laser such that the intensity of one or both of the beams is at a maximum. U.S. Pat. No. 4,152,071, which issued May 1, 1979 to T. J. Podgorski and is assigned to the assignee of the present invention, illustrates a control mechanism and circuitry as just described. Path length transducers for controlling the path length of the ring laser are well known, and particularly described in U.S. Pat. No. 3,581,227, which issued May 25, 1971 to T. J. Podgorski, also assigned to the assignee of the present invention, U.S. Pat No. 4,383,763, which issued May 17, 1983 to Hutchings et al and U.S. Pat. No. 4,267,478, which issued May 12, 1981 to Bo H. G. Ljung, et al. All these patents are incorporated herein by reference.
In the aforementioned patents, the beam intensity is either detected directly as illustrated in the aforementioned patents, or may be derived from what is referred to as the double beam signal such as that illustrated in U.S. Pat No. 4,320,974, which issued on Mar. 23, 1982 to Bo H. G. Ljung, and is also incorporated herein by reference.
In path length control systems of the prior art, the path length controller finds mirror positions for which the lasing polygon path length, i.e., the ring laser path length, is an integral number of wavelengths of the desired mode or frequency, as indicated by a spectral line, of the lasing gas.
Herein mode is defined as the equivalent of one wavelength of the laser beam. For a helium-neon laser, one mode is equal to 0.6328 microns which is equal to 24.91 microinches. With proper design, the path length control forces the path length traversed by the laser beams to be a value which causes the laser beams to be at maximum power.
Path length control functions may be differentiated between different phases of laser gyro start up. Initially when the laser is started the path length controllers are used to help guide the laser into a useful mode that provides good dynamic response for other active parameters of the laser gyro, such as laser output, gyro dither and bias control. After start up, path length control helps compensate for temperature and aging effects as described above.
In prior art designs, start up path length control was accomplished with the aid of a predetermined set point of the pick off voltage and the use of a voltage sweep. The desired set point was specified when the laser gyro was constructed. The laser gyros of the prior art had difficulty adjusting to two common effects, temperature fluctuations and fluctuations in system response due to aging. Therefore it is the motivation of the invention to provide a dynamic compensation mechanism capable of acquiring a particular laser mode, calculating volts per mode, and changing laser modes.